Presently, there are several methods available for cloning mammalian genes. The standard approach to cloning mammalian genes requires obtaining purified protein, determining a partial amino acid sequence of the purified protein, using the partial amino acid sequence to produce degenerate oligonucleotide probes, and screening cDNA libraries with these probes in order to obtain cDNA encoding the protein. This method is time consuming and, because of the degeneracy of the probes used, may identify sequences other than those encoding the protein(s) of interest. Many mammalian genes have been cloned this way, including the cGMP phosphodiesterase expressed in retina (Ovchinnikov, Y-A. et al., FEB 223: 169 (1987)).
A second approach to cloning genes encoding a protein of interest is to use a known gene as a probe to find homologs. This approach is particularly useful when members of a gene family or families are sufficiently homologous. It is reasonable to expect that members of a given gene family can be so cloned once one member of the family has been cloned. The D. melanogaster dunce phosphodiesterase gene was used, for example to clone rat homologs. (Davis, R. L. et al., Proc. Natl. Acad. Sci. USA 86: 3604 (1989); Swinnen, J. V. et al., Proc. Natl. Acad. Sci. USA 86: 5325 (1989)). Although members of one family of phosphodiesterase genes might be cloned once a member of that family has been cloned, it is unclear whether the nucleotide sequences of genes belonging to different phosphodiesterase gene families exhibit sufficient homology to use probes derived from one family to identify members of another family.
It would be useful to have a method which could be used to clone genes which does not have the limitations of presently available techniques.